The invention relates to communication systems, and more particularly, to techniques for applying forward error correction to frames of a synchronous optical communication system.
In general, the noise associated with a communication channel can cause transmission errors. The error probability of a digital communication system is the number of errors per total number of bits received. For instance, if one error bit per 100 bits occurs, the error probability is {fraction (1/100)}, or 10xe2x88x922. Forward error correction (FEC) techniques, such as Reed-Solomon or BCH encoding, employ the concept of redundancy in order to maintain an acceptable error rate. In this context, redundancy is the fraction of total message content that can be eliminated without loss of essential information. A FEC code is an encoding algorithm that uses more signal elements (e.g., redundant bits) than necessary to represent the essential information to be communicated. The intended receiver can decode the transmitted information (including both essential information and redundant elements) using a complementary FEC decoding algorithm to recover the intrinsic information. By using redundancy, the probability of error is spread out over a greater number of signal elements, and a more robust communication is generally achieved.
FEC and Optical Transmission
Historically, the use of forward error correction (FEC) in the context of transmitting optical signals has been limited. FEC has been used, for instance, in submarine systems to decrease the number of necessary 3R (re-time, re-shape, re-transmit) regenerators for transoceanic transmission of optical signals. Recommendation G.975 of the Telecommunications Standards Section of the International Telecommunication Union (ITU-T) addresses this particular application. FEC is also being used in the context of dense wavelength division multiplexing (DWDM) optical systems. By using FEC in such systems, it is possible to increase the number of the transmitted colors significantly thereby resulting in an increase in the overall transmission rate of the optical system.
Recommendation G.975 Coding
Recommendation G.975 defines the architecture of an encoder. The encoder produces FEC frames. A digital de-multiplexer and a multiplexer are used before and after the encoding stage, respectively. The multiplexer effects the interleaving operation. The de-multiplexer, on the other hand, provides a complementary de-interleaving operation needed for data integrity. The corresponding decoder architecture uses the same digital multiplexer and digital de-multiplexer. The input of the decoder is passed through a de-multiplexer to implement de-interleaving stage resulting in N parallel signals. After passing these signals through a bank of N RS decoders, they are multiplexed back to the original signal. A typical value for N, for example, is 16 corresponding to an interleaving of depth sixteen.
In applying Recommendation G.975, a coding gain of about 6 dB can be achieved. This coding gain translates into transforming a signal with a bit error ratio (BER) of about 10xe2x88x924 to a signal with a BER of about 10xe2x88x9215. The input signal to the encoder is a multiple of STM-16 (synchronous transport module level 16) where each STM-16 is processed in parallel. A SMT-16 frame has a bit rate of 2.48832 gigabits-per-second (Gbps). The input signal (referred to as payload or STM-16 data) is first segmented into blocks of 238 bytes each. After adding an overhead byte, the resultant 239 bytes are passed through a Reed-Solomon (255,239) encoder thereby generating FEC frames of 255 bytes each. Based on the properties of Reed-Solomon (RS) redundancy codes, the added 16 bytes of parity can correct up to eight erroneously received byte-symbols within that FEC frame.
If the number of byte-symbols in error in a received FEC frame is greater than eight, then the decoder is not able to correct the errors and decoding failure is declared. To reduce such xe2x80x9cburst errors,xe2x80x9d it is therefore necessary to use interleaving where bytes from each FEC frame are spread during transmission so that the probability of receiving more than eight bytes in error for each FEC frame is minimized. Recommendation G.975 proposes blocks of N consecutive FEC frames where the first column of the frames is transmitted first followed by the second column, and so on. Using an interleaving of depth N, it is possible to correct burst error lengths of up to 8N bytes. The penalty paid for this error correction capability is the extra 255*N bytes of delay introduced by the interleaver.
Optical Transport Network (OTN)
In general, optical networks operate to provide transport, multiplexing, routing, supervision, and survivability of client signals that are processed in the physical domain. The optical transport network (OTN) initiative of the ITU-T (as well as the T1X1.5 technical subcommittee) uses the extra overhead byte added to each FEC frame to propose a new digital hierarchy (e.g., ITU-T G.709) and the corresponding network architecture (e.g., ITU-T G.872). To form an OTN frame, the interleaving depth is set to 16 to arrive at a FEC super-frame. This super-frame has 16 bytes of overhead followed by 3808 bytes of payload. The tail end of this super-frame includes 256 parity bytes. Four super-frames are used to form an OTN frame. The OTN frame is defined for three different line rates k (where k=1, 2 and 3, corresponding to line rates of 2.5, 10 and 40 Gbps, respectively). Note that the format of an OTN frame is the same for all the line rates, but the frame rate depends on the line rate. For purposes of clarity, this line rate k is distinct from and not to be confused with the pre-encoding block length k, which is used in reference to the selected coding algorithm. Synchronous Optical Network (SONET)
One type of client signal in an OTN is generally referred to as synchronous optical network (SONET). SONET is defined by a set of American National Standards Institute (ANSI) standards defining the rates and formats for synchronous optical networks. Such standards include ANSI T1.105, ANSI T1.106 and ANSI T1.117. In addition, Telecordia Technologies"" GR-253-CORE standard provides generic requirements on SONET transport systems. Similar standards, referred to as Synchronous Digital Hierarchy (SDH), have been established by the ITU-T (e.g., G.707). Note that SONET and SDH are technically compatible standards having differences that are readily understood by one skilled in the art. The SONET frame rate is always the same (e.g., 8000 frames/sec) regardless of the line rate.
SONET""s synchronous, flexible, optical hierarchy allows many signals of different capacities to be carried by a transmission system. This is accomplished by a byte-interleaved multiplexing scheme. Generally, byte-interleaving simplifies multiplexing, and facilitates end-to-end network management. The first step in the SONET multiplexing process involves the generation of the lowest level or xe2x80x9cbasexe2x80x9d signal. In SONET, this base signal is referred to as synchronous transport signal level-1 (STS-1).
The STS-1 signal operates at a bit rate of 51.84 Megabits per second (Mbps). The bit rate of each of the higher level signals is the signal integer level number multiplied by 51.84 Mbps. Thus, a hierarchal family of STS-N signals provided as illustrated by Table 1. An STS-N signal is formed from N byte-interleaved STS-1 signals. The base signal for a SDH system is referred to as synchronous transport module level-1 (STM-1), which has a bit rate of 155.52 Mbps. The hierarchal family of STM-M signals is also illustrated by Table 1. Note that N typically ranges from 1 to 192, where only particular values are recognized (e.g., 1, 3, 12, 48, and 192). However, higher values of N, as well as other recognized values are possible. Note the relationship of M to N (M equals N divided by 3).
As can be seen in Table 1, each of the digital signals (e.g., DSn) received by a SONET system is associated with a particular bit rate which dictates how many of that signal type can be associated with a particular STS level including necessary overhead added by the SONET system. The digital signal received by the SONET system is generally referred to as payload. The optical form of each STS-N signal is referred to as OC-N (the optical carrier signal for that level). The STS and OC signals of any one level have the same bit rate. Note that SDH systems do not distinguish between the logical signal (STM-M) and the physical signal (OC-M). As the definition of STS-1 effectively defines the entire hierarchy of synchronous optical signals, the discussion of FIG. 1 is presented in terms of STS-1. The bit rate of a higher level STS-N signal is equal to N times the rate of STS-1. Similarly, the bit rate of an STM-M of a SDH system is equal to M times the bit rate of the STM-1 signal.
Generally, an STS-N signal includes two parts: the STS payload and the STS overhead. The STS payload carries the information portion of the signal, and the STS overhead carries signaling and protocol information. As a transmission signal enters a SONET system, that signal is converted from its current user format to STS format. The terminating equipment of the SONET""s receiving node converts the STS-N signal back to the original user format. Note that synchronous transport module level 1 (STM-1) is the lowest bit rate signal for SDH at 155.52 Mbps, which happens to be equivalent to STS-3.
General Overview of SONET System
FIG. 1 illustrates a conventional SONET system. The terminating equipment designated with an xe2x80x9caxe2x80x9d (e.g., 105a, 110a, 115a) represent a transmitting node of the SONET system, and the terminating equipment designated with a xe2x80x9cbxe2x80x9d (e.g., 105b, 110b, 115b) represent a receiving node of the SONET system. In this sense, an end-to-end SONET system is illustrated. The payload is received by the PTE 105a. The payload can be, for example, DS1, DS2, DS3, DS4NA, video or other digital service signals (whether synchronous or asynchronous). For purposes of discussion, assume that the payload is made up of 28 DS1 signals, each DS1 signal having a bit rate of 1.544 Mbps for a total bit rate of 43.232 Mbps. PTE 105a maps the DS1 signals and added path overhead into a format required by the line layer. This format is referred to as a synchronous transport signal level 1 synchronous payload envelope (STS-1 SPE). The STS-1 SPE is provided to the LTE 110a. The LTE 110a maps the STS-1 SPE and added line overhead into a line layer signal. This resulting line layer signal is then provided to the STE 115a of the section layer, which maps the line layer signal and added section overhead into an STS-1 signal. The output STE 115a is a logical signal, which can be converted to its physical equivalent (e.g., an optical signal or OC-1) for communication over the communication channel.
The receiving node essentially provides complementary functions to those of the transmitting node. Assuming the signal was transmitted in its physical form, it is converted back to its logical form (e.g., STS-1) at the physical layer. At the section layer, the STS-1 signal is then de-mapped by STE 115b into section layer signal and section overhead. At the line layer, the section layer signal is de-mapped by the LTE 110b into the SPE and line overhead. At the path layer, the SPE is de-mapped by the PTE 105b into the payload (and path overhead) that was originally received by PTE 105a 
Path Terminating Equipment and Path Layer
PTE 105a and 105b are network elements that originate (105a) and terminate (105b) payload channels such as DSn channels. Additionally, PTE 105a and 105b can originate, access, modify, terminate or otherwise process the path overhead, or can perform any combination of these actions. Path overhead is assigned to and transported with the payload until the payload is de-multiplexed. Generally, path overhead supports functions that are necessary to transport the payload, such as performance monitoring and automatic protection switching. Virtual container path overhead is the equivalent for a SDH system. The path layer, therefore, facilitates the reliable transport of payloads between PTE 105a and 105b. In addition, the path layer maps payloads into the format required by the line layer. The path layer communicates end-to-end by virtue of the path overhead, and provides error monitoring and connectivity checks. Note that PTE 105a and 105b are implemented in conventional technology, such as fiber optic terminating systems.
Line Terminating Equipment and Line Layer
LTE 110a and 110b are network elements that originate (110a) and terminate (110b) line signals. Additionally, LTE 110a and 110b can originate, access, modify, terminate or otherwise process the line overhead, or can perform any combination of these actions. Generally, line overhead supports functions such as locating the SPE in the frame (synchronization), multiplexing or concatenating signals, performance monitoring, automatic protection switching, and line maintenance. LTE 110a and 110b are implemented in conventional technology, such as add/drop multiplexers or digital cross-connect systems. The line layer (also referred to as the multiplex section), therefore, facilitates the reliable transport of path layer payload and its overhead across the physical communication channel. All lower layers (e.g., section layer) exist to provide transport for this layer. Synchronization and multiplexing functions are also provided for the path layer.
Section Terminating Equipment and Section Layer
STE 115a and 115b are network elements that originate (115a) and terminate (115b) section signals such as STS-N or OC-N signals. Additionally, STE 115a and 115b can originate, access, modify, terminate or otherwise process the section overhead, or can perform any combination of these actions. Generally, section overhead supports functions such as framing, scrambling, performance monitoring, and section maintenance. STE 115a and 115b are implemented in conventional technology, such as 3R regenerators. The section layer (also referred to as the regenerator section), therefore, facilitates the reliable transport of an STS-N (or OC-N or STM-M) frame across the physical communication channel. This layer uses the physical layer for transport. Functions of this layer include framing, scrambling, section error monitoring, and section level communications overhead (e.g., local orderwire).
FIG. 2 illustrates the operation of a SONET system having transmitting and receiving nodes, and a repeater to regenerate the transmitted signal. The repeater is an STE node that only terminates the section layer, and its purpose is to perform 3R regeneration as well as performance line monitoring.
Transport Overhead Byte Designations of SONET System
With the optical interface layers hierarchy of a SONET system in mind, it is now appropriate to consider the overhead associated with these layers. FIG. 3 illustrates the transport overhead byte designations of a SONET system in accordance with the ANSI T1.105 standard, which is herein incorporated by reference in its entirety. In general, the functions assigned to the section and line layers are combined into a structure of 27 bytes called the transport overhead. Each of the bytes is assigned to a particular functional layer and a position in the overhead columns of the STS-1 frame. Typically, nine of the 27 bytes are overhead for the section layer, 18 bytes are overhead for the line layer, and the transport overhead occupies the first three columns of the STS-1 frame. The overhead associated with a given layer is modified, created or otherwise processed by the equipment associated with that layer before that overhead is inserted in the outgoing signal.
Section Layer Overhead Bytes
The framing bytes A1 and A2 are allocated in each STS-1 for framing. Note these framing bytes are not scrambled thereby allowing the individual frames to be identified at the receiving node. The section Trace byte J0 (in the first STS-1 of the STS-N) is generally used to send section trace messages. The section BIP-8 (bit interleaved parity-8) byte B1 is located in the first STS-1 of an STS-N, and is used for a section error monitoring function. The Orderwire byte E1 is located in the first STS-1 of an STS-N, and is used for a Local Orderwire (LOW) channel. A LOW channel is used for voice communication between regenerators, hubs and remote terminal locations. The section User Channel byte F1 is located in the first STS-1 of an STS-N, and is used by the network provider. The section Data Communication Channel bytes D1, D2 and D3 are located in the first STS-1 of an STS-N, and are used for section data communications. These three bytes effectively provide a message-based channel (192 kilobits per second) for alarms, maintenance, control, monitoring, administering and other communication needs between section terminal equipment. This channel is used for communicating internally generated, externally generated and supplier-specific messages.
Line Layer Overhead Bytes
The payload pointer bytes H1 and H2 are allocated to a pointer that indicates the offset in bytes between the pointer and the first byte of the STS SPE. The pointer bytes are typically used in all STS-1s within an STS-N to align the STS-1 transport overheads in the STS-N, and to perform frequency justification. These bytes can also be used to indicate concatenation, and to detect path Alarm Indication Signals (AIS-P). The Pointer Action byte H3 is allocated for SPE frequency justification purposes. The H3 byte is used in all STS-1s within an STS-N to carry the extra SPE byte in the event of a negative pointer adjustment. The line BIP-8 byte B2 is allocated in each STS-1 for a line error monitoring function. Generally, the N line BIP-8 bytes in an STS-N electrical or OC-N signal form a single error monitoring facility capable of measuring bit error rates (up to 10xe2x88x923) independent of the value of N. The APS Channel bytes K1 and K2 are located in the first STS-1 of an STS-N, and are used on the protection line for Automatic Protection Switching (APS) signaling between line terminating equipment that uses line level protection switching (e.g., in systems using linear APS, or in Bi-directional Line Switched Rings). The K2 byte is also used to detect line AIS (AIS-L) and line Remote Defect Indication (RDI-L) signals.
The line Data Communication Channel bytes D4 through D12 are located in the first STS-1 of an STS-N, and are used for line data communication. These nine bytes effectively provide a message-based channel (576 kilobits per second) for alarms, maintenance, control, monitoring, administering and other communication needs. This channel is used for internally generated, externally generated and supplier-specific messages. The Synchronization Status byte S1 is located in the first STS-1 of an STS-N. Bits 5 through 8 of this byte are allocated to convey the synchronization status of the corresponding network element. The REI-L bytes M0 and M1 are allocated for a Line Remote Error Indication (REI-L) function (given an electrical physical signal) that conveys the error count detected by line terminating equipment (using the line BIP-8 code) back to its peer line terminating equipment. The Orderwire byte E2 is located in the first STS-1 of an STS-N, and is used for an Express Orderwire (EOW) channel between line entities.
The overhead fields and their respective descriptions for an OTN frame are defined in ITU-T G.709. Many of the SONET management capabilities are being duplicated within the OTN environment. For example, path and section error monitoring (PM and SM), general communication channel (GCC) and automatic protection switching (APS) are also being provisioned in SONET. Thus, in protecting SONET frames with FEC using OTN frame format, many of the management functionalities of SONET are duplicated and not utilized. This situation could be avoided by using a SONET framing scheme rather than a separate framing scheme such as the OTN frame format.
What is needed, therefore, are techniques for protecting SONET frames with FEC using SONET framing. Such techniques could also be employed for protecting SDH frames with FEC using SDH framing.
Techniques for applying forward error correction to optical communication signals such as those of SONET and SDH systems are provided. Existing framing structures associated with the particular optical communication system and protocol are utilized, and a separate framing for the forward error correction framing is not required. The techniques are independent with respect to the particular error correction coding method being used. Moreover, established management infrastructure associated with the given communication system can be used to manage the encoded data. Thus, a separate management layer is not necessary.
The features and advantages described herein are not all inclusive and, in particular, many additional features and advantages will be apparent to one of ordinary skill in the art in view of the drawings, specification, and claims. Moreover, it should be noted that the language used in the specification has been principally selected for readability and instructional purposes, and not to limit the scope of the inventive subject matter.